docs: def notify(self, value: Any) -> asyncio.Future:
          """ Only notifies subscribers if state has changed. See also
          Publisher.notify().
  
          :param value: value to be emitted to subscribers
          :returns: a future if at least one subscriber has returned a future,
              elsewise None
          """
          if self._state == value:
              return None
  
          self._state = value
          return Publisher.notify(self, value)